Combined combustion and steam turbine power plant

ABSTRACT

A combined combustion and steam turbine power plant which includes a steam turbine unit, a boiler unit for supplying steam to the steam turbine unit, and a combustion turbine unit with an exhaust gas duct structure for supplying the turbine exhaust gases to the boiler unit, has at least one heat recovery steam generator arranged in the exhaust gas flow downstream of the boiler unit and connected to the steam-turbine unit so as to supply any steam generated in the heat recovery steam generator to the steam turbine unit.

This is a continuation of application Ser. No. 08/008,023 filed Jan. 25,1993, now U.S. Pat. No. 5,375,410.

BACKGROUND OF THE INVENTION

The present invention resides in a combined cycle power plant,particularly an integrated steam and combustion turbine power plant withhighly efficient Brayton/Rankine cycle arrangement.

The efficiency of conventional steam power plants has been improved byintegration of a combustion turbine into conventional steam turbineplants with gas, oil or coal fired boilers which include combustion airpreheaters in their exhaust duct to recover as much as possible heatfrom the boiler exhaust by transferring it to the combustion air for theboiler. However, in a combined cycle plant, combustion air preheatersare not needed since the boilers receive the hot exhaust gases of theassociated gas turbines as combustion gases for the boilers.

In order to recover the energy in the boiler exhaust in such combinedarrangements, generally, a portion or all of the feedwater of theintegrated steam cycle is passed through a stack gas cooler which isinstalled downstream of the boiler. Often a boiler by-pass is providedby way of which the exhaust gases from the gas turbine can by-pass theboiler when the load on the boiler is relatively low. However, allexhaust gas passes through the stack gas cooler which is locateddownstream of the juncture where any exhaust gas by-passing the boileris recombined with the boiler discharge gas. The stack gas cooler istherefore always exposed to the full gas turbine exhaust flow unless thestack gas cooler includes a controllable by-pass flow structure.

It is to be noted that single, large industrial combustion turbines haveonly a limited capability of adjusting their volumetric flow rate. Thus,the energy available from the stack gas cooler does not vary much overthe plant load range. However, the feedwater flow through the stack gascooler decreases substantially as plant load is reduced. Therefore, evenwith a slight reduction in plant load the temperature of the feedwaterin the stack gas cooler may reach its saturation point so that steamingmay occur as the feedwater flows from the stack gas cooler to theboiler. To avoid this undesirable situation, full load feedwater flowthrough the stack gas cooler is generally maintained at all times and,during part load, when the boiler requires only part of the feed water,the balance is dumped into the condenser. The energy of the feedwaterdumped into the condenser is not recovered but is lost which results inreduced operating efficiencies.

It is therefore the principal object of the present invention to providea combined steam and combustion turbine power plant in which theefficiency does not suffer during part load operation as a result ofcombustion turbine excess exhaust heat generation.

SUMMARY OF THE INVENTION

In a combined combustion and steam turbine power plant which includes asteam turbine unit, a boiler unit for supplying steam to the steamturbine unit, a combustion turbine unit with an exhaust gas ductstructure connected to the boiler unit for supplying the hot turbineexhaust gases thereto, and a stack connected to the boiler unit so as toreceive the exhaust gases therefrom for discharge into the atmosphere,at least one heat recovery steam generator is arranged in the exhaustgas flow downstream of the boiler unit and connected to the turbine unitso as to supply any steam generated in the heat recovery steam generatorto the turbine unit.

With the arrangement according to the invention, all of the availablestack gas energy of power generating plants with integrated combustionturbines and conventionally fired boilers can be removed even duringpart load operation. Also, any concerns with regard to steaming in stackgas coolers are eliminated.

BRIEF DESCRIPTION OF THE DRAWINGS

The sole FIGURE is a schematic representation of the combined steam andcombustion turbine power plant according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

As shown in the FIGURE, a combined steam and combustion turbine powerplant comprises a gas turbine-generator section 1 which is connected toa steam generating section 2 that receives the hot combustion gases fromthe gas turbine and converts energy contained in the combustion gasesinto steam. The steam augmented generally by a boiler burning additionalfuel is supplied to a steam turbine-generator section 3.

As shown in the FIGURE, the steam turbine-generator section 3 includes ahigh-pressure turbine 4, an intermediate pressure turbine 5 and a lowpressure-turbine 6 all coupled to a generator 7. The steam, afterpassage through the various turbines, is condensed in a condenser 8 andthe condensate is then preheated in stages in feedwater heaters 9, 10,11, 12, 13, 14, 15 and 16 which receive steam from the various stages ofthe various turbines. The steam turbine-generator arrangement isessentially as described in U.S. Pat. No. 5,140,818 which is assigned tothe assignee of the present invention.

The steam generating section 2 comprises a duct structure 17 whichincludes various heat exchangers and a boiler 18 and terminates in anexhaust stack 19 through which the exhaust gas from the gas turbinesection 1 is finally discharged.

Under full power operation, most of the high pressure steam foroperating the high pressure turbine 4 is generated in the boiler 18. Thecondensate from the steam turbine section 3 is returned to theeconomizer 20 of the boiler 18. The steam generated in the boiler 18 iscollected in the boiler drum 21 and is then supplied to the highpressure turbine 4 via a superheater 22 within the boiler 18. The steamdischarged from the high pressure turbine 4 is returned to a reheater 23which is disposed in the boiler 18, in which the steam is reheated foradmission to the intermediate pressure turbine 5. A second reheater 24is arranged in the duct 17 at the entrance end adjacent the gas turbinesection 1 in parallel flow arrangement with the first reheater 23 so asto permit reheating of the high pressure steam by both of the tworeheaters 23, 24.

Adjacent the exhaust stack 19, the duct structure 17 includes heatrecovery steam generators (HRSG) 25 and 26 for cooling the exhaust gas.As shown in the FIGURE, two HRSG's are utilized so as to be operative atdifferent pressures and temperatures. The higher temperature HRSG 26 isarranged in the duct structure 17 upstream of-the lower temperature HRSG25 and supplies the steam generated therein via a steam superheater 27and the reheater 24 as drive steam to the intermediate pressure turbine5. The lower temperature HRSG 25 supplies the steam generated thereinvia steam heater 28a arranged in the duct structure 17 upstream of theHRSG 26 and a steam heater 28b arranged upstream of the boiler 18 to thelow pressure steam turbine 6. Both HRSGs 25, 26 are supplied withcondensate from condenser 8 via a final heat exchanger 29 from which thecondensate flows either directly to the lower temperature HRSG 25 or viaanother heat exchanger 30 arranged just downstream of the highertemperature HRSG 26 to the HRSG 26. Additional condensate can becirculated through economizer 29 and delivered back to the feedwaterheater system as indicated in FIG. 1 to thereby further improve cycleperformance.

The HRSGs 25 and 26 are capable of recovering any available amount ofheat from the exhaust gas before it is discharged through the stack 19.They are each operated at essentially constant temperature and maintainan essentially constant exhaust gas temperature as the steam generatedtherein is utilized in the steam turbines.

As shown in the FIGURE, there are preferably provided two HRSGs, thesecond HRSG 25 being arranged in the combustion gas duct just downstreamof the HRSG 26 and operated at a lower temperature so as to further coolthe combustion gas leaving the first HRSG 26.

Ahead of the boiler 18, the duct structure 17 may include a turbineexhaust gas cooler in the form of another HRSG 31 in order to reduce theexhaust gas temperature to a temperature more easily accommodated byconventional boiler structures. Its feedwater and steam system arearranged essentially in parallel with the boiler 18 and connected so asto receive its feedwater from the highest temperature feedwater heater16 and to supply its steam to the high pressure turbine 4 via asuperheater 32 arranged in the duct structure 17 adjacent the combustionturbine unit 1.

Depending on the steam load required, the temperature of the exhaust gasfrom the combustion turbine 1 will be increased as it enters the boiler18 by combustion of boiler fuel 33 supplied thereto. Also, additionalfresh air may be supplied to the duct 17 for combustion of the fuel, viafresh air supply means 34. Under certain load conditions, the boilerstructure can be by-passed by the turbine exhaust gas by appropriatepositioning of a damper 35 which is arranged in the duct structurepivotally so as to direct the turbine exhaust gas either through awindbox 36 to the boiler 18 or to by-pass the boiler.

The steam flow to and the water flow from the steam turbine and theboiler as suggested herein is by way of piping arranged externally ofeither of those components so that the addition of a HRSG(s) to aconventional plant does not change the configuration or the thermalcycle of a conventional plant.

The configuration according to the invention provides not only forimproved efficiency at full load operation but for even greaterefficiency during part-load operation where, for plant load reduction,full load may be maintained for the combustion turbine while fuelcombustion in the boiler may be reduced. In fact, efficiency continuesto improve until combustion of fuel in the boiler is terminated at whichpoint operation of the plant corresponds to the operation of a normalcombined cycle power plant. Further load reduction is obtained bypart-loading the combustion turbine. Over the entire range of part loadoperation however the arrangement according to the present inventionprovides for maximum cycle efficiency as the heat available from theexhaust gas can always be recovered by the HRSGs and utilized in powergeneration. No dumping of feedwater flow into the condenser to avoidsteaming in a stack gas cooler is necessary.

We claim:
 1. A combined cycle power plant, comprising:a steam turbine; aboiler unit for generating and supplying a first source of steam to saidsteam turbine unit; a combustion turbine unit with an exhaust gas ductstructure for supplying the combustion turbine exhaust gases to saidboiler unit and an exhaust gas stack for discharging the exhaust gasesinto the atmosphere, said duct structure including at least one heatrecovery steam generator disposed in the flow of exhaust gas dischargedthrough said exhaust gas stack, said heat recovery steam generator beingconnected to said steam turbine unit for generating and supplying asecond source of steam to said steam turbine unit; a boiler fuel supply;and means for combusting said boiler fuel supply for increasing theboiler temperature independent of the temperature of the exhaust gasesfrom said combustion turbine.
 2. A power plant according to claim 1,wherein said steam turbine unit comprises a high pressure, anintermediate pressure and a low pressure turbine and said heat recoverysteam generator is connected to said intermediate pressure turbine.
 3. Apower plant according to claim 2, wherein said heat recovery steamgenerator is connected to said intermediate pressure turbine via a steamsuperheater arranged in the flow of said combustion gas upstream of saidheat recovery steam generator.
 4. A power plant according to claim 3,wherein another heat recovery steam generator is disposed in the flow ofexhaust gas downstream of said one heat recovery steam generator and isoperated at a temperature and pressure lower than said one heat recoverysteam generator, the other heat recovery steam generator being connectedto said low pressure turbine for delivering its steam to said lowpressure turbine.
 5. A power plant according to claim 4, wherein saidother heat recovery steam generator is connected to said low pressureturbine via at least one superheater arranged in said exhaust gas flowupstream of said one heat recovery steam generator.
 6. The power plantaccording to claim 1, wherein said duct structure further comprises adamper for controlling the interaction of the exhaust gases from saidcombustion turbine with said boiler.